The VLA-COSMOS 3 GHz Large Project: The infrared-radio correlation of star-forming galaxies and AGN to z ≲ 6

Delhaize, Jacinta and Smolčić, Vernesa and Delvecchio, I. and Novak, Mario and Sargent, M. and Baran, N. and Magnelli, B. and Zamorani, G. and Schinnerer, E. and Murphy, E. J. and Aravena, M. and Berta, S. and Bondi, M. and Capak, P. and Carilli, C. and Ciliegi, P. and Civano, F. and Ilbert, O. and Karim, A. and Laigle, C. and Le Fevre, O. and Marchesi, S. and McCracken, H. J. and Salvato, M. and Seymour, N. and Tasca, L. (2017) The VLA-COSMOS 3 GHz Large Project: The infrared-radio correlation of star-forming galaxies and AGN to z ≲ 6. Astronomy and Astrophysics, 602. Art. No. A4-17. ISSN 0004-6361

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We examine the behaviour of the infrared-radio correlation (IRRC) over the range 0 <z ≲ 6 using new, highly sensitive 3 GHz observations with the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory in the 2 deg2 COSMOS field. We distinguish between objects where emission is believed to arise solely from star-formation, and those where an active galactic nucleus (AGN) is thought to be present. We account for non-detections in the radio or in the infrared using a doubly-censored survival analysis. We find that the IRRC of star-forming galaxies, quantified by the infrared-to-1.4 GHz radio luminosity ratio (qTIR), decreases with increasing redshift: qTIR(z) = (2.88 ± 0.03)(1 + z)− 0.19 ± 0.01. This is consistent with several previous results from the literature. Moderate-to-high radiative luminosity AGN do not follow the same qTIR(z) trend as star-forming galaxies, having a lower normalisation and steeper decrease with redshift. We cannot rule out the possibility that unidentified AGN contributions only to the radio regime may be steepening the observed qTIR(z) trend of the star-forming galaxy population. We demonstrate that the choice of the average radio spectral index directly affects the normalisation, as well as the derived trend with redshift of the IRRC. An increasing fractional contribution to the observed 3 GHz flux by free-free emission of star-forming galaxies may also affect the derived evolution. However, we find that the standard (M82-based) assumption of the typical radio spectral energy distribution (SED) for star-forming galaxies is inconsistent with our results. This suggests a more complex shape of the typical radio SED for star-forming galaxies, and that imperfect K corrections in the radio may govern the derived trend of decreasing qTIR with increasing redshift. A more detailed understanding of the radio spectrum is therefore required for robust K corrections in the radio and to fully understand the cosmic evolution of the IRRC. Lastly, we present a redshift-dependent relation between rest-frame 1.4 GHz radio luminosity and star formation rate taking the derived redshift trend into account.

Item Type: Article
Keywords: galaxies: evolution, galaxies: star formation, radio continuum: galaxies, infrared: galaxies
Date: 13 June 2017
Subjects: NATURAL SCIENCES > Physics > Astronomy and Astrophysics
Additional Information: © 2017 ESO. Received 29 July 2016. Accepted 17 February 2017. Published online 13 June 2017. This research was funded by the European Unions Seventh Frame-work programme under grant agreement 337 595 (ERC Starting Grant, “CoSMass”). This research was supported by the Munich Institute for Astroand Particle Physics (MIAPP) of the DFG cluster of excellence “Origin and Structure of the Universe”. N.B. acknowledges the European Unions Seventh Framework programme under grant agreement 333 654 (CIG, AGN feedback). M.B. and P. Ciliegi acknowledge support from the PRIN-INAF 2014. A.K. acknowledges support by the Collaborative Research Council 956, sub-project A1, funded by the Deutsche Forschungsgemeinschaft (DFG). M.T.S. acknowledges support from a Royal Society Leverhulme Trust Senior Research Fellowship. Support for B.M. was provided by the DFG priority programme 1573 “The physics of the interstellar medium”. M.A. acknowledges partial support from FONDECYT through grant 1 140 099.
Divisions: Faculty of Science > Department of Physics
Publisher: EDP Sciences
Depositing User: Vernesa Smolčić
Date Deposited: 20 Nov 2017 10:07
Last Modified: 20 Nov 2017 10:07

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